1. Field of the Invention
The present invention relates to the measurement of certain physical properties of fluids and, more particularly, to the determination of density or specific gravity of gases as a function of both the specific heat and thermal conductivity. In a preferred embodiment, a trapped gas sample transmits steady state any transient responses to input energy of limited duration which can be measured electrically as by extracting the influence of the input energy in the form of measurable change in temperature of an appropriate sensor in contact with the gas of interest.
2. Prior Art
In the prior art, the traditional approach to determining specific heat, c.sub.p, has been via calorimetry using reversible step increases of energy fed to a thermally isolated or adiabatic system. Such devices are bulky, slow and cumbersome. Little progress has been made toward the automation of a rapid method to make this determination.
With respect to measuring thermal conductivity in fluids, various types of detectors have been used. This includes resistance bridge type sensors. One such device is described in U.S. Pat. No. 4,735,082 in which thermal conductivity is detected using a Wheatstone bridge technique in which a filament in one diagonal of the bridge is placed or positioned in a cavity through which the sample gas of interest is passed. The filament is used to introduce a series of amounts of thermal energy into the fluid of interest at alternating levels by varying the input voltage which, are, in turn, detected at the other diagonal as voltage difference signals. Integration of the changes of the value of the successive stream of signals yields a signal indicative of the heat dissipation through the fluid, and thus, the thermal conductivity of the fluid.
Further to the measurement of thermally induced changes in electrical resistance, as will be discussed in greater detail below, especially with reference to prior art FIGS. 1-5, recently very small and very accurate "microbridge" semiconductor chip sensors have been described in which etched semiconductor "microbridges" are used as condition or flow sensors. Such sensors might include, for example, a pair of thin film sensors around a thin film heater. Semiconductor chip sensors of the class described are treated in a more detailed manner in one or more of patents such as U.S. Pat. Nos. 4,478,076, 4,478,077, 4,501,144, 4,651,564 and 4,683,159, all of common assignee with the present invention.
It is apparent, however, that it has been necessary to address the measurement of specific heat c.sub.p, and thermal conductance, k, of a fluid of interest with separate and distinct devices. Not only is this quite expensive, it also has other drawbacks. For example, the necessity of separate instruments to determine specific heat and thermal conductivity may not allow the data consistency and accuracy needed for useful fluid process stream (gas or liquid) characterization. The required degree of correlation may not be present. Because the determination of density as contemplated herein depends on both measurements, this takes on even more importance.